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CSC 774 Advanced Network Security

CSC 774 Advanced Network Security. Topic 4. Broadcast Authentication. What Is Broadcast Authentication?. One sender; multiple receivers All receivers need to authenticate messages from the sender. Challenges in Broadcast Authentication.

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CSC 774 Advanced Network Security

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  1. CSC 774 Advanced Network Security Topic 4. Broadcast Authentication Dr. Peng Ning

  2. What Is Broadcast Authentication? • One sender; multiple receivers • All receivers need to authenticate messages from the sender. Dr. Peng Ning

  3. Challenges in Broadcast Authentication • Can we use symmetric cryptography in the same way as in point-to-point authentication? • How about public key cryptography? • Effectiveness? • Cost? • Research in broadcast authentication • Reduce the number of public key cryptographic operations Dr. Peng Ning

  4. CSC 774 Advanced Network Security Topic 4.1 TESLA and EMSS Dr. Peng Ning

  5. Outline • Two Schemes • TESLA • Sender Authentication • Strong loss robustness • High Scalability • Minimal overhead • EMSS • Non-Repudiation • High loss robustness • Low overhead Dr. Peng Ning

  6. TESLA - Properties • Low computational overhead • Low per packet communication overhead • Arbitrary packet loss tolerated • Unidirectional data flow • No sender side buffering • High guarantee of authentication • Freshness of data Dr. Peng Ning

  7. TESLA – Overview • Timed Efficient Stream Loss–tolerant Authentication • Based on timed and delayed release of keys by the sender • Sender commits to a random key K and transmits it to the receivers without revealing it • Sender attaches a MAC to the next packet Pi with K as the MAC key • Sender releases the key in packet Pi+1 and receiver uses this key K to verify Pi • Need a security assurance Dr. Peng Ning

  8. Ki’=F’(Ki) TESLA – Scheme I • Each packet Pi+1 authenticates Pi • Problems? • Security?Robustness? Dr. Peng Ning

  9. TESLA – Scheme I (Cont’d ) • If attacker gets Pi+1 before receiver gets Pi, it can forge Pi • Security Condition • ArrTi + t < Ti+1 • Sender’s clock is no more than t seconds ahead of that of the receivers • One simple way: constant data rate • Packet loss not tolerated Dr. Peng Ning

  10. TESLA – Scheme II • Generate a sequence of keys { Ki } with one-way function F • Fv(x) = Fv-1( F(x) ) • Ko = Fn (Kn) • Ki = Fn-i(Kn) • Attacker cannot invert F or compute any Kj given Ki, where j>i • Receiver can compute all Kj from Ki, where j < i • Kj = Fi-j (Ki); K’i = F’ (Ki) Dr. Peng Ning

  11. TESLA – Scheme II (Cont’d) Dr. Peng Ning

  12. TESLA – Scheme III • Remaining problems with Scheme II • Inefficient for fast packet rates • Sender cannot send Pi+1 until all receivers receive Pi • Scheme III • Does not require that sender wait for receiver to get Pibefore it sends Pi+1 • Basic idea: Disclose Ki in Pi+d instead of Pi+1 Dr. Peng Ning

  13. TESLA – Scheme III (Cont’d) • Disclosure delay d = (tMax + dNMax)r • tMax: maximum clock discrepancy • dNMax: maximum network delay • r: packet rate • Security Condition: • ArrTi + t < Ti+d • Question: • Does choosing a large d affect the security? Dr. Peng Ning

  14. TESLA – Scheme IV • Deals with dynamic transmission rates • Divide time into intervals • Use the same Kito compute the MAC of all packets in the same interval i • All packets in the same interval disclose the key Ki-d • Achieve key disclosure based on intervals rather than on packet indexes Dr. Peng Ning

  15. TESLA – Scheme IV (Cont’d) Dr. Peng Ning

  16. TESLA – Scheme IV (Cont’d) • Interval index: i = (t – To)/T • Ki’ = F’(Ki) for each packet in interval i • Pj = < Mj, i, Ki-d, MAC(Ki’, Mj) > • Security condition: • i + d > i’ • i’ = (tj+ t-To)/T • i’ is the farthest interval the sender can be in Dr. Peng Ning

  17. TESLA – Scheme V • In Scheme IV: • A small d will force remote users to drop packets • A large d will cause unacceptable delay for fast receivers • Scheme V • Use multiple authentication chains with different values of d • Receiver verifies one security condition for each chain Ci, and drops the packet is none is satisfied Dr. Peng Ning

  18. TESLA--Immediate Authentication • Mj+vd can be immediately authenticated once packet j is authenticated • Not to be confused with packet j+vd being authenticated Dr. Peng Ning

  19. TESLA – Initial Time Synchronization • RS: Nonce • S R: {Sender Time tS, Nonce, …}Ks-1 R only cares the maximum time value at S. Max clock discrepancy: T = tS-tR Dr. Peng Ning

  20. EMSS • Efficient Multichained Streamed Signature • Useful where • Non Repudiation required • Time synchronization may be a problem • Based on signing a small number of special packets in the stream • Each packet linked to a signed packet via multiple hash chains Dr. Peng Ning

  21. EMSS – Basic Signature Scheme Dr. Peng Ning

  22. EMSS – Basic Signature Scheme (Cont’d) • Sender sends periodic signature packets • Pi is verifiable if there exists a path from Pi to any signature packet Sj Dr. Peng Ning

  23. EMSS – Extended Scheme • Basic scheme has too much redundancy • Split hash into k chunks, where any k’ chunks are sufficient to allow the receivers to validate the information • Rabin’s Information Dispersal Algorithm • Some upper few bits of hash • Requires any k’ out of k packets to arrive • More robust Dr. Peng Ning

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